Frequency modulation radio receiver



Patented Feb. 9, 1943 was UNITE snares TNT FREQUENCY MODULATION RADIO RECEIVER Claudius '1. McCoy, Philadelphia, and Palmer M. Craig, Cheltenham, Pa., assignors to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application October 21, 1941, Serial No. 415,966

12 Claims. (Cl. 250--) lation receivers, however, there has been pos- T sible up to the present no such simple method of providing a tuning indicator'to determine when the set is properly tuned to the incoming carrier wave.

It is very desirable in a frequency modulation receiver to avoid the reproduction of amplitude modulation appearing on an incoming carrier, since none of the intelligence of the Wave will be in the form of amplitude modulation. Such amplitude modulation as does exist will consist largely of extraneous noise, distortion, or interference. When a frequency modulation receiver employing a balanced detector is properly tuned to an incoming wave, only the frequency modulation on the carrier will be effectively detected, since in a balanced detector the effects of am plitude modulation are largely concelled or neutralized in the output circuit of the detector. If, however, the frequency modulation receiver is not properly tuned to the incoming carrier frequency, the amplitude modulation on the Wave will be reproduced in the output system of the receiver, and the greater the degree of mistuning, the more effective the set will be as an implitude modulation receiver and, therefore, the less desirable its performance.

There is one serious difiiculty which complicates the tuning of frequency modulation receivers, which is not experienced in the tuning of amplitude modulation receivers. In amplitude modulation receivers, there is normally present a single relatively sharp resonance peak, and it is a fairly simple matter to tune the receiver accurately to this peak, even without a tuning aid. In tuning to an F. M. signal with an F. M. receiver, however, it will be found that there are three resonance peaks rather than one, and that these peaks are very broad by comparison with the sharp peak encountered in A. M. systems. Consequently, the tuning of an F. M. receiver by an unskilled individual may prove to be an exceedingly difficult operation. Yet accurate tuning is of more importance in an F. M. receiver than it is in an A. M. receiver, since the characteristic of a balanced frequency detector which permits it to discriminate against noise, or against amplitude modulation in general, depends, as'is well understood, upon the incoming F. M; signal being accurately tuned or centered with respect to the center-frequency of the detector, circuit. In general the noise reduction properties of balanced frequency-detectors decrease rapidly on either side of center of the middle-of the three peaks If the receiver is tuned to either of the outer two of these peaks, there'will besubstantially no noise reduction whatever. These outer peaks appear as a result of detection on the outer slopes of the detector characteristic, and in practice may be separated from the desired peak by more than kilocycles.

With the foregoing in mind, one object of the present invention is to provide a novel tuning indicator for a frequency modulation receiver.

Another object of the invention is to provide a tuning indicator which will not require additional tubes.

Stillanother object of the invention is to provide accurate tuning by audible means.

A further objectiof the invention is to provide accurate tuning of a frequency modulation re,- ceiver by amplitude modulating the received carrier with alocal signal, and observing the response of the receiver to said signal while tuning the receiver. ji

. Other objects of the invention will be apparent from the accompanying drawing and the following description which are to be taken as merely exemplary of the invention and not as limiting the same.

In the drawing:

Fig. 1 is a block diagram of an F. M. receiver, of the superheterodyne type, incorporating my invention; and

Fig. 2 is a wiring diagram of part of the receiver shown in Fig. 1.

Referring first to Fig. 1 there is shown a superheterodyne receiver for frequency modulated waves, which comprises a radio frequency amplifier orantenn-a tuning stage i connected to a I pick-up device for radio waves, e. g. a dipole, or other suitable antenna 2. Signal from stage I is supplied to the converter 3 which also receives the signal from oscillator 4, aswell understood. Tuning may be effected by the usual ganged tuning condensers 5 and 6. The I. F. signal from converter 3 goes through the first intermediate frequency amplifier 1, the second intermediate frequency amplifier 8, the balanced detector '9, the audio frequency amplifier H), and finally to the loud speaker or other converting device H.

As thus far described, this is a conventional frequency modulation receiver. According to the present invention, there is shown connected to the second intermediate frequency amplifier 8 a source of audio frequency voltage I2 which supplies modulating voltage to the incoming wave and enables tuning of the receiver in the manner hereinafter described.

The invention may be clearly understood by reference to the detailed illustration of Fig. 2, in which there is shown part of the frequency modulation receiver which includes in its intermediate frequency amplifier system an amplifier tube I3 of the conventional pentode type. The control grid of the tube is connected to the output of the previous tuned intermediate frequency circuit which in frequency modulation receivers is commonly tuned to 4.3 megacycles. of the tube may be connected through a voltage dropping resistor I4 to the source of high voltage plate current. A filter condenser I5 may be connected from the screen grid to ground or to chassis. The anode of the pentode I3 is shown connected through the primary of the coupling transformer I6 to the plate supply source. The primary of transformer I6 is tuned to the intermediate frequency by condenser I7. The secondary of transformer I6 is center-tapped and has its ends connected, respectively, to the anodes of the detector diodes I8 and I9. Connected between the mid-tap of the secondary coil and the junction of load resistors 20 and 2I is a parallel resonant circuit 22. This resonant tuned circuit is advantageously adjusted to the same intermediate frequency, e. g. 4.3 megacycles. Preferably, the untuned secondary of transformer I6 is coupled tightly to the primary circuit.

The tuned circuit 22 is loosely coupled to the primary of transformer I6. The opposite ends of resistors 20 and 2I are connected to the oathodes of the diodes I8 and I9, and the lower end of resistor 2| is grounded. By-passing the resistors 20 and 2| for radio frequencies are the condensers 23 and 24. The audio signal'appearing across resistors 2|] and 2I is supplied to the audio amplifier via the D. C. blocking condenser 25. A detailed description and explanation of this balanced detector circuit is to be found in the copending application of C. T. McCoy, Serial No. 419,200, filed November 14, 1941, but since the present invention is adapted for use with bal anced detectors generally, it is not deemed necessary to explain the operation of any specific balanced detector here.

Supplying the radio receiver with high voltage direct current is a power supply circuit having a transformer 26 with a primary winding 21 for connection with A. 0. power mains, a low voltage winding 28 for heating the cathode of a rectifier tube 29, and a high voltage winding 30 which has a grounded center tap, as shown. The plates of the rectifier tube are connected to the ends of the high voltage winding. Connected to the rectifier cathode through the conventional filter network 3|, 32, 33 is the high-voltage direct-current con- The screen grid tween the screen grid of tube I3 and a selected point on the high voltage winding of transformer 26, e. g., one extremity of said winding. By this arrangement, accurate tuning of the receiver is accomplished, as described hereinafter.

In the operation of the receiver, a frequency modulated signal is tuned in by varying the ganged tuning condensers 5 and 6 (Fig. 1), resulting in a signal being imposed on the control grid of the intermediate frequency tube I3. If the receiver is properly tuned, the frequency of the signal in the intermediate frequency amplifier will exactly correspond to the center frequency of the balanced detector, e. g. 4.3 megacycles. By the term center frequency as applied to the balanced detector in a superheterodyne receiver for receiving frequency modulated signals is meant the frequency of the unmodulated carrier wave, as converted to the intermediate frequency, and is the frequency at which the two branches of the detector respond equally and oppositely to the said unmodulated carrier wave. If the receiver is mistuned to the incoming signal, the intermediate frequency signal will be above or below the center frequency of the balanced detector. As previously mentioned, a balanced detector rejects amplitude modulation when the receiver is correctly tuned to the carrier frequency, but responds thereto when the receiver is incorrectly tuned. Advantage is taken of this, according to the present invention, by introducing amplitude modulation of substantially constant value on the incoming wave, and then tuning the receiver until the effects of the amplitude modulation appearing in the output of the receiver are a minimum.

When a frequency modulated signal is received. the switch 36 is closed, thus impressing an alternating voltage, derived from the transformer 26, on the screen grid of the intermediate frequency tube I3. The imposition of this voltage on the screen grid element of the tube produces amplitude modulation of the frequency-modulated wave, and there appears in the detector output circuit, if the receiver is mistuned, a hum voltage proportional to the degree of mistuning. The receiver is then tuned until the hum decreases to a minimum level. The receiver is then properly tuned, and the switch 36 may be opened.

It will be apparent that the alternating voltage derived from the power supply, as described above, will contain various harmonics of the supply line frequency, e. g. 60 cycles. The elements I4, I5, and 31 form a frequency selective network, and by assigning proper values to these elements, selected harmonics may be employed to amplitude modulate the received signal. Since the human ear is more sensitive to the harmonic frequencies than to the very low fundamental frequency of the supply line, the audible tuning may be facilitated by utilizing certain of the harmonic frequencies thereof.- Thus, if the fundamental frequency is cycles, it may be desired to accentuate, for example, the third and fourth or higher harmonics, in which case the network I4--I 535--3I may be designed accordingly. Since elements I4 and I5 are the usual screen grid resistor and radio frequency by-pass condenser, respectively, the harmonic selection is achieved particularly by elements 35 and 31. By assigning a relatively small value of capacity to condenser 35 and a relatively large value of resistance to resistor 31, the selection of harmonics may be accomplished.

In the illustrated embodiment, the receivers power supply is used as a source of the audio frequency modulating voltage. but it is to be understood that a separate oscillator may be used, and may be connected to amplitude-modulate the incoming signal, e. g. by connection to the screen grid or other control element of an intermediate frequency tube. Moreover, instead of employing audible tuning as described, it is possible to use a visual tuning indicator which will be operative when the amplitude modulation is applied and which will show visually when the amplitude modulation output is a minimum. By making the tuning indicatorrespond only to the frequency of the modulating voltage which is introduced, it will be affected only by the introduced amplitude modulation, and not by intermittent variable modulation which might be present from interference.

It is to be understood, therefore, that the invention is not limited to the specific embodiment illustrated, but is capable of various modifications.

We claim:

1. In a frequency modulation radio receiver, means for tuning the receiver to a desired frequency-modulated carrier signal, frequency-detector means characterized in that said means is substantially non-responsive to amplitude modulation components of the carrier signal when the receiver is accurately tuned but is responsive to said components when the receiver is inaccurately tuned, means operable at will for applying an amplitude-modulation component to the received carrier signal while tuning the receiver, and means for observing the response of said detector means to said amplitude-modulation component, whereby the receiver may be accurately tuned to the desired carrier by adjusting the tuning until the response of said detector means to said applied amplitude-modulation component is a minimum.

2. In a frequency modulation radio receiver, means for tuning the receiver to a desired frequency-modulated carrier signal, frequency-detector means characterized in that said means is substantially non-responsive to amplitude modulation components of the carrier signal when the receiver is accurately tuned but is responsive to said components when the receiver is inaccurately tuned, a source of audio frequency voltage, means operable at will for amplitude-modulating the received carrier signal with said voltage while tuning the receiver, and means for observing the response of said detector means to said voltage, whereby the receiver may be accurately tuned to the desired carrier by adjusting the tuning until the response of said detector means to said voltage is a minimum.

3. In a frequency modulation radio receiver, power supply means for the receiver, means for tuning the receiver to a desired frequency-modulated carrier signal, frequency-detector means characterized in that said means is substantially non-responsive to amplitude modulation components of the carrier signal when the receiver is accurately tuned but is responsive to said components when the receiver is inaccurately tuned, means operable at will for deriving a signal from said power supply means and for amplitudemodulating the received carrier signal with said derived signal while tuning the receiver, and means for observing the response of said detector means to said derived signal, whereby the receiver may be accurately tuned to the desired carrier by adjusting the tuning until the response of said detector means to said derived signal is a minimum.

4. In a frequency modulation radio receiver, a carrier frequency channel, a balanced frequency-detector coupled to said channel, means for tuning said receiver to a desired frequencymodulated carrier signal, said balanced detector being relatively insensitive to amplitude modulation components of the carrier signal when the receiver is accurately tuned but responding to said components when the receiver is inaccurately tuned, a source of a signal of predetermined frequency, means in said carrier frequency channel for amplitude-modulating the carrier signal with said last-named signal, and means for observing the relative response of said balanced detector to the amplitude modulation components of the carrier signal thereby to tune the receiver accurately to the desired carrier signal.

5. In a frequency modulation radio receiver, a carrier frequency channel having at least one screen grid tube, a balanced frequency-detector coupled to said channel, means for tuning said receiver to a desired frequency-modulated carrier signal, said balanced detector being relatively insensitive to amplitude modulation components of the carrier signal when the receiver is accurately tuned but responding to said components when the receiver is inaccurately tuned, a source of alternating voltage of audible frequency, means including a switch for connecting said source to the screen grid of said tube, and means for observing the relative response of said balanced detector to the amplitude modulation components of the carrier signal thereby to tune the receiver accurately to the desired carrier signal.

6. In a superheterodyne radio receiver adapted to receive frequency-modulated signals, an intermediate frequency amplifier including at least one multi-grid tube, a balanced detector circuit which is substantially unaffected by amplitude modulation on an incoming wave only when the frequency of the incoming carrier signal in the intermediate frequency amplifier is the same as the center frequency of the balanced detector circuit, a source of voltage of audio frequency, and switch means for connecting said source of voltage to a grid of said tube so as to amplitude modulate an incoming signal and cause an audible tone on said signal of appreciable magnitude if the incoming carrier does not coincide with said center frequency. I

7. In a superheterodyne radio receiver adapted to receive frequency-modulated signals, an intermediate frequenc amplifier, a balanced detector circuit which is substantially unaffected by amplitude modulation on an incoming wave When the frequency of the carrier signal in the intermediate frequency amplifier is the same as the center frequency of the balanced detector circuit but which is otherwise affected by amplitude modulation, a source of voltage of frequency such 9. In a radio receiver adapted to receive irequency-modulated signals, a detector which is substantially unafi'ected by amplitude modulation on an incoming signal when the receiver is properly tuned to an incoming signal frequency, means for amplitude-modulating the incoming signal with an audible tone, and means for observing the effect of said amplitude modulation on said detector, whereby the point of proper tuning may be determined by the minimum reproduction of said tone.

10. The method of accurately tuning a frequency modulation receiver, which consists in applying an amplitude modulation component to the received frequency-modulated carrier signal, passing said signal through a balanced detector, and observing the response of said detector to said component while tuning the receiver to the carrier signal until said response is a minimum.

11. The method of accurately tuning a receiver for frequency-modulated signals, which consists in amplitude-modulating the received frequencymodulated signal with an audio tone voltage, applying said amplitude-modulated signal to a balanced detector, observing the efiect of said amplitude modulation on said balanced detector, and tuning said receiver until said efiect is a minimum.

12. The method of accurately tuning a receiver of the superheterodyne type adapted to receive frequency-modulated signals, which consists in converting an incoming frequency-modulated carrier signal to a frequency-modulated signal having a predetermined intermediate frequency, amplitude-modulating said last-named signal with an audio tone voltage, applying said amplitude-modulated signal to a balanced detector, observing the efiect of said amplitude modulation on said balanced detector, tuning said receiver until the effect of said amplitude modulation is a minimum, and removing said amplitude modulation from said signal.

CLAUDIUS T. MCCOY. PALMER M. CRAIG. 

